Thermocouple casing



April 20, 1926.

P. BROWN THERMOCOUPLE CASING Filed Nov. 30, 1925 INVENTOR W W WWW 3. [WWATTORNEY Patented Apr. 20, 1926.

UNITED STATES PATENT OFFICE.

RICHARD :9. BROWN,

PENNSYLVANIA.

0F PHILADELPHIA, PENNSYLVANIA, ASSIGNOR TO THE BROWN INSTRUMENT COMPANYOF PHILADELPHIA, PENNSYLVANIA,

A CORPORATION OE Application filed November 30, 1925. Serial No. 72,237.

To all whom it may concern:

Be it known that I, RioHAm) P. BROWN, a citizen of the United States,and resident of Philadelphia, Philadelphia County, and State ofPennsylvania, have invented certain new and useful Improvements inThermocouple Casings, of which the following 15 a specification.

The present invention comprises an enclosing tube or casing for athermo-couple which is of novel composition. The invention was devisedwith the object of provid ing a satisfactory and durable casing for athermocouple immersed in a bath of a hot molten metal such as lead, orthe zinc containing alloy used in making zinc die castings. Such a metalbath corrosively attacks thermo-couple casings formed of the metalsheretofore employed for the purpose with such rapidity as to make thelife of such a tube quite, short.

In accordance with the present invention, I form the portion of thecasing of a. thermocouple to be immersed in a bath of molten metal ofthe kind referred to, out of a chromium iron alloy of the samecomposition as the so-called stainless iron now in extensive use becauseof its resistance to surface oxidation under certain conditions which,however, are very different from those attending the use of athern'io-couple casing inserted in molten lead and zinc baths.

The composition of a chromium iron alloy which has given excellentresults in practice as a casing material for a thermo-couple used asabove described is as follows: carbon 10%, silica 20%, manganese .30%,

- chromium 13.00%, iron 86.40%.

chromium alloy referred to may be employed in casings of various forms.A desirable mode of forming-such a casing consists in drilling an axialhole of suitable diameter in a bar. of the alloy, the hole extendingfrom one end of the bar nearly but not quite to the opposite end.

The accompanying drawing is a somewhat conventional sectional elevationof a melting pot with a thermo-cou-ple mounted therein.

In the drawing A represents the melting pot proper, containing a. body Bof molten metal such aslead or the zinc alloy employed The iron in theformation of the so-called zinc die castings. As shown, the pot A ismounted in a heating chamber or furnace C. The thermocouplestructure'eomprises a tubular casing element D which is verticallydisposed and extends down into the molten metal bathB and a horizontalportion D to which the upper end of the section D is connected as bymeans of an elbow fitting. The section D extends horizontally throughthe wall of the chamber 0. At its outer end the section D is connectedto the usual thermo-couple casing head D provided with terminals D forconnecting the external leads (not shown) to the thermo-coupleconductors d, which are united to form a thermo-couple hot junctionlocated within the casing section D adjacent the closed lower end of thelatter. The casing section D may be formed as described by drilling ahole in a bar of the iron chromium alloy described above, the holeextending downward nearly to but not quite to the lower end of the bar.

In practical use I have found that a thermo-eouple casing section D ofthe kind illustrated in the drawing and formed of the iron chromiumalloy referred to above will, when used in a bath of molten lead, or abath of the alloy used in forming so-called zinc die castings, have alife many times greater than that of any material heretofore employed inthe formation of thermo-couple casings. The invention is particularlyuseful in maintaining the proper temperature of a. molten lead bathemployed in hardening and heat treating dies. For this purpose thetemperature of the bath is in the neighborhood of 1500 F., and at thattemperature molten lead attacks and rapidly destroys iron and ironnickel alloys and other materials heretofore employed in formingthermo-couple casings. The corrosive action of the lead is augmented, Ibelieve, by the presence of other substances as zinc, ordinarily presentin small amounts in such lead baths. The alloys used in themanufactureof so-called zinc die castings usually contain approximately 5% ofaluminum, approximately 3% of copper, and approximately 92% of zinc.This alloy at the temperature rapidly attacks and destroys iron, andiron and nickel alloys, and other materials heretofore employed inltheformation of thermocouple casings. A thermo-couple tube formed of theiron chromium material specified above in accordance with the presentinvention and used in lead baths described above, ordinarily has aneffective life of many months.

The use of the improved. thermo-couple casing tubes is desirable, ofcourse, because it reduces the frequency with which such tulbes.

must be renewed. It is also durable because the failure of theprotective casings occasionally results in the destruction of thethermo-couple elements which may be quite expensive, and furthermore,the failure of a thermo-coupleoin use from any cause may have seriousconsequences because of the resultant deleterious effect ontheexpensivedies being hardened and heat treated, or on the character of thecastings produced.

and zinc alloy Having now described my invention what I claim as new anddesire to secure by Letters Patent is:

1. A thermo-couple casing formed of an iron chromium alloyiresistant tothe corrosive action of molten lead and zinc.

RICHARD P. BROWN.

2. A thermo-couple casing formed of an

